1. Field of the Invention
This invention relates to a method of making a tubular member, and particularly concerns a metallic shell for an ignition plug, a glow plug, an oxygen concentration sensor, a water temperature sensor, and a knocking sensor.
2. Description of Prior Art
In a gasoline engine, an ignition plug is placed within a recess provided with a cylinder head of the engine. The ignition plug has a metallic shell which has a hexagonal portion, an outer diameter of which is smaller than that of a middle portion of the metallic shell.
One example of this metallic shell is disclosed in E.P. Publication No. 0036050 in which a blank is extruded to form a first tubular end circular in section, an outer diameter of which is smaller than that of a middle portion of the blank. Then, the blank is reduced at its upper tubular portion to make a second tubular end hexagonal in section, an outer diameter of which is smaller than that of a middle portion of the blank, but greater than that of the first tubular end. The blank is further transferred to next station in which the blank is reduced. During this transferring process, the blank is inverted upside down drawing in an arc with the blank held by fingers. During the inverting process, it is feared that the blank would collide on a die to do damage on the blank or the die which composes a cold extrusion machine.
Further, in the case in which the fingers moves the blank at a high speed, it is feared that the blank would fly out of the fingers by a centrifugal force.
In order to avoid this inconvenience, it is necessary to move the blank at such a speed that the fingers can continue to hold the blank, thus leading to a deteriorated production speed.
Therefore, it is an object of the invention to provide a method of making a tubular member which is capable of avoiding a damage by collision, and enhancing a transferring speed to lead to an improved production.
On the other hand, as shown in FIG. 22 and 23, a punch 210 is forced into a recess 201 of a blank 200 to extrude an upper tubular portion 202 hexagonal in section within an annular space 221 between a mandrel 220 and a vertical hole 231 of a die 230. During this process, a great amount of friction heat is generated between an inner wall of the recess 201 and an outer surface of the mandrel 220. In order to alleviate the friction heat, a cooling liquid oil supplied between the mandrel 220 and tubular portion 202. The fluid oil flowing between the recess 201 and the mandrel 220, however, is unable to find an escape path because the extrusion process is carried out before the blank is penetrated all through the length by a punching process. The fluid oil pressure exerted between the mandrel 220 and tubular portion 202 works to deform the inner wall of the tubular portion 202 to define an escaping passage or sink mark as designated, for example, by numeral 204 in FIG. 24 which is a sectional view taken along line A--A of FIG. 23.
This deformation of the tubular portion 202 causes to appear a shrinkage on a surface of the metallic shell so as to deteriorate an appearance of the shell which is regarded as an unacceptable product when a rear end of the shell is secured to an insulator of an ignition plug by means of caulking at an assemble process.
Therefore, it is another object of the invention to provide a method of making a tubular member which is capable of preventing the tubular member from being deformed due to a fluid pressure between a mandrel and an upper tubular portion of the tubular member.